Diverse sulfate- and sulfur intermediate- reducing bacteria contributing to annually expanding water anoxia in an oil sands pit lake.

Acidification and dissolved oxygen (DO) depletion in mining-affected water bodies are global environmental challenges associated with the oxidation of reduced S species (S), including sulfide and sulfur oxidation intermediate compounds (SOI). The recent detection of diverse SOI species in tailings impoundments and pit lakes highlights overlooked risks from microbial S cycling, that are difficult to geochemically quantify due to rapid S turnover. This study characterizes sulfur-reducing bacteria (SRB; capable of sulfate and/or SOI reduction), in Base Mine Lake (BML), the first oil sands tailings reclamation end pit lake. Integrating field physicochemical, geochemical, 16S rRNA sequencing, and metagenomic results, we identified seasonal and DO-dependent variations in SRB communities that were associated with the spring-summer physicochemical and [S] changes. Specifically, late summer stratified BML water column evidenced DO-dependent SRB zonation, initiating with strictly anaerobic sulfate-reducing bacteria in bottom anoxic waters, and SRB SOI-reduction/disproportionation processes extending S impacts into the suboxic regions. The identification of SRB communities with diverse S reduction/disproportionation capabilities reveal the escalating environmental implications of annually recurrent and expanding sulfur cycling to BML water DO persistence. Findings here inform future oil sands tailings reclamation regulatory assessment, and provide essential insights for S biogeochemical cycling and outcomes in mining-impacted water contexts.